For many years I rode a 1981 Honda CBX, this is a air-cooled inline 6 with 24 valves and six carbs. They were notorious for getting lousy gas mileage, usually from the mid twenties to the mid thirties. They also required frequent valve adjustments and carburator synchronozations. I used MMO in this bike consistently at a one-ounce-per-gallon ratio, even carried it with me on long road trips in a bottle with measurements on the side (thank you for the new clear bottles!).
Anyhow, I regularly got mileage in the mid forties on my bike. And all the folks at the ICOA (International CBX Owners Association) I rode with doubted what I said. Then one day coming home from a weekend at Morro Bay CA, a few of us were riding togother. We rode about 100 miles. They all filled up before we left, I started with 20 miles on a tank. When we got to the fuel stop (because they were all running low), mine took almost a gallon less than everyone elses. Everyone noticed and asked about that, upon which I told them the same thing I had always told them, MMO at one-ounce-per-gallon.
About those valve adjusts and carb syncs. I put 50,000 miles on the bike before I sold it, after owning it for 16 years. During that time I had to adjust valves one time, for one loose valve. I never had to sync the carbs, the gauges always told me it was within parameters. I have always felt the MMO contributed to this fantastic performance, and now use it in every vehicle I own.
My ultimate vehicle, a 2004 Ford LIGHTNING gets 10 to 12 mpg. Really, when one buys a big truck with a Roots-blown V-8, you should not expect great mileage. I have been using MMO in it for awhile now without checking my mileage. I will do this on my next fill-up, and see what it shows. I am not concerned about it, but it would still be cool to see a 2 mpg increase.